Cameron backs Blair on Iraq war
David Cameron said it was important to do what you think is right
In an interview for BBC's Friday Night with Jonathan Ross, he said the war had been "very unpopular" and some bad decisions had been made since it began.
But Mr Cameron said "those of us who supported" the military action should "see it through".
He praised Tony Blair's reform of the Labour party but said he wanted the Tories to be "the party of the future".
Mr Cameron was also quizzed about his attitude towards drugs and he insisted he would not consider legalisation.
He said: "I would change drugs policy by putting a much greater emphasis on treatment, and a much greater on education."
On the issue of Iraq, he told Ross he supported Mr Blair's decision to go to war.
"The world has got smaller and we have to recognise that what happens in other countries has a bearing on us," he said.
He added: "You've got to do what you think is right even if it's unpopular, that's the only thing you can do."
The party voted for change by voting for me
Mr Cameron also said the prime minister had made "some great steps" in foreign affairs, but said there was "more of a question mark" over domestic issues.
"There are things that we'd keep and build on," he said.
He accused Labour of losing touch and said he wanted the Conservatives to offer a "real alternative" at the next general election.
"Sometimes it's uncomfortable, but I think we're making progress. The party voted for change by voting for me," he said.
"It's a process of change - the environment campaign really shows that."
The Tory leader admitted he found Prime Minister's Questions "very nerve-wracking" the first time he was involved as Leader of the Opposition and he described it as a "circus".
He also paid tribute to the former prime minister Lady Thatcher and said she had given the country back the chance of a growing economy.
He said she advised him to "get a good night's sleep, eat well and read all the papers".
Friday Night with Jonathan Ross will be broadcast on BBC1 at 2235 BST.